1. Field of the Invention
This invention pertains to a multiple heating zone process and apparatus for fabricating core/clad optical glass fibers, particularly from thermally unstable glass.
2. Description of Related Art
Although this discussion will focus on the chalcogenide glasses, it should be understood that this invention pertains to other glasses as well, including oxide and fluoride glasses, which are thermally unstable in that crystallization occurs during fiber fabrication thereof during cooling of the core to the draw temperature through the crystallization temperature.
Silica glass has phonon energy of about 1100 cm−1, fluoride glass has phonon energy of about 560 cm−1, and chalcogenide glass has phonon energy of about 425 cm−1 and lower. It is chalcogenide glass that is often used to make glass fibers for infrared applications, due to its low phonon energy and its spectral range of 2–12 microns.
Chalcogenide glasses and optical fibers made therefrom can transmit light in the 2–12 micron region, depending upon their composition. Chalcogenide glasses are made of at least one chalcogen element, i.e., sulfur (S), selenium (Se), tellurium (Te), and at least one other element such as arsenic (As), germanium (Ge), gallium (Ga), antimony (Sb), indium (In), cadmium (Cd), etc. Many applications are associated with these glasses and optical fibers, such as high power laser delivery, laser surgery, medical diagnostics, remote chemical sensing, near field microscopy, infrared imaging, etc. In addition, because the phonon energy of these glasses is lower than oxide and fluoride glasses, chalcogenide glasses have been considered as host materials for rare earth doped optical fiber lasers and fiber amplifiers, operating in the IR region. For fiber lasers and fiber amplifiers, the core glass should contain sufficient amount of at least one rare earth element, such as praseodymium (Pr), neodymium (Nd), dysprosium (Dy), etc., to make optical devices Unfortunately, rare earth elements are not soluble in most stable chalcogenide glasses, such as arsenic sulfide (As40S60) or arsenic selenide (As40Se60). Other chalcogenide glasses, such as gallium-containing sulfide or selenide glasses that can dissolve sufficient amount of rare earth elements, are not thermally stable and have a tendency to crystallize during the fiber drawing process. Therefore, a fiber drawing technique is needed to enable fabrication of rare earth doped, core/clad optical fibers made, from chalcogenide and other glasses by maintaining the core at a temperature below its crystallization temperature Tx.
High quality, low loss chalcogenide optical fibers are needed for the applications mentioned above. The main sources for scattering optical loss are impurity particles in the glass itself and formation of crystals, bubbles, or core/clad interface defects during the fiber fabrication process. U.S. Pat. No. 5,879,426 explains the double crucible process for making chalcogenide optical fibers. That process is suitable for thermally stable chalcogenide glasses such as, arsenic sulfide and arsenic selenide glasses, that do not crystallize during the re-melting and subsequent cooling to the fiber drawing temperature Td. Arsenic sulfide fibers drawn from that process have shown a minimum loss of 0.1 dB/m. Using that technique for fabricating optical fibers from less thermally stable chalcogenide Lasses, such as GeGaAsS or GeGaAsSe, especially when these glasses are doped with rare earth elements, is not possible because they will crystallize during the slow cooling process from their melting temperature to the drawing temperature. Therefore, a unique process and apparatus are needed to overcome the crystallization problem capable of fabricating core/clad glass fibers while maintaining the temperature of the core glass below its crystallization temperature.